The representative uses of an industrial paper include honeycombs, electrical insulation papers, separators, filters, etc. The physical properties required for the industrial paper are electrical insulation, mechanical property, light weight, uniformity, porosity, etc., depending on applications. However, above all these requirements, it should have superior paper strength and paper formation property.
As a representative example, a honeycomb using an aramid fiber as an industrial paper is prepared from an aramid paper containing a para-aramid fiber, a pulp and another fibrous material and further containing a binder or a resin coat. As a representative example of the binder used in the preparation of the aramid paper, U.S. Pat. No. 6,551,456B describes that a fibrous paper may be prepared by using a polyester fiber as a binder for an aramid pulp and the paper can improve the impregnation property of a resin for a thermosetting structure. And, KR10-2009-0091811A describes a technology of preparing an aramid paper by using a thermoplastic fiber having a melting temperature above the curing temperature of a matrix resin and a glass transition temperature above 100° C. as a binder. As a representative example of preparation of a paper for a honeycomb through resin coating, KR10-2010-0094543A describes a technology of preparing a paper for a honeycomb from a meta-aramid fibrid using a phenol-, polyimide- or epoxy-based coating agent.
However, because the aramid paper for a honeycomb according to the prior art contains, in addition to the aramid pulp or fibrid, a binder formed of a thermoplastic fiber or resin, which has a weaker strength than the aramid, there is the problem of decreased mechanical property of a base paper or difficulty in reducing weight because of decreased porosity. In addition, there are the problems of poor transfer of a base paper, nonuniformity of strength and aggregation between flocs.
Meanwhile, for an electrical insulation paper used in mobile phones, power cables, etc., heat resistance is required additionally. Therefore, there have been many research and development efforts recently on a sheet formed of an aromatic polyamide (aramid) pulp or fiber as an insulation sheet having superior electrical insulation property and heat resistance and preparation and application thereof. However, because a sheet consisting only of the aramid pulp or fiber has poor flexibility and strength in general, there have been research and development efforts to improve the flexibility and strength by blending the aramid fiber with another fiber by using a binder. In this regard, JP 2535418B discloses an aramid insulation paper wherein aramid and polyester fibers are blended to reduce flexural rigidity. Although flexibility is ensured through this, electrical insulation property and heat resistance are unsatisfactory. Although JP 5591046B improves the heat resistance to some extent by blending polycarbonate and aramid fibers, uniform electrical insulation property is not ensured as compared to the insulation paper consisting only of aramid.
Recently, KR10-2014-0040096A disclosed a technology of preparing an electrical insulation paper by mixing 40-100 wt % of a film-type para-aramid fibrid with an inorganic filler, etc. using a binder via a jet spin process. And, KR10-2014-0038935A disclosed a technology of preparing an electrical insulation paper using an aramid microfilament and an aramid fibrid or pulp as a non-resin-type binder.
However, the electrical insulation paper prepared according to the prior art contains, in addition to the aramid pulp or fibrid, the binder formed of the thermoplastic fiber, inorganic filler, etc. In particular, for the electrical insulation paper, a technology for solving the problems of poor transfer of a base paper, nonuniformity of strength and aggregation between flocs by improving the paper formation property, which is more important for uniform electrical insulation property than the improvement of the strength of the base paper, has not been proposed.
Meanwhile, because it is difficult to achieve satisfactory physical properties with a single layer of an aramid short fiber only, a technology for preparing a laminate using an aramid paper and a polymer is being developed. As a representative example, KR10-2005-0071531A describes a technology of forming an aramid paper through calendering and applying thereon a polyester-based polymer or copolymer.
However, because the aramid paper prepared according to the prior art contains, in addition to the aramid pulp or fibrid, the binder formed of the thermoplastic fiber, inorganic filler, etc., there may be difference in physical properties such as a thermal expansion coefficient, electrical conductivity and thermal conductivity. Accordingly, it is not applicable to a material or a component which requires precision. In addition, it cannot solve the problems of poor transfer of a base paper, nonuniformity of strength and aggregation between flocs.